This invention relates to a golf ball having improved resilience.
In the related art, ionomer resins are widely used as the cover material of golf balls. Ionomer resins are ionic copolymers between olefins (such as ethylene) and unsaturated carboxylic acids (such as acrylic acid, methacrylic acid and maleic acid), wherein some of the acidic groups are neutralized with metal ions such as sodium, lithium, zinc and magnesium. On account of excellent properties including durability, resilience, and scuff resistance, ionomer resins are advantageously used as the base resin of cover materials, and actually become the mainstream of the current cover resins. Although the rebound of ionomer resins is considerably high, the customers are eager to play with golf balls of higher resilience and better flight performance.
One method for improving the resilience of golf balls using ionomer resins as the cover material is disclosed in JP-A 4-156865. A golf ball is improved in resilience by covering a core with an ionomer resin and annealing the ionomer resin under suitable conditions to modify the crystallographic state thereof. The annealed ionomer resin exhibits such a crystal melting behavior that a first melting peak appears at 85 to 95xc2x0 C. and a second melting peak appears at 70 to 80xc2x0 C. when measured by a differential scanning calorimeter. More particularly, the ionomer resin as applied onto the core is annealed at 45 to 63xc2x0 C. for about 3 to 240 hours. Then the crystallographic state of the ionomer resin is changed whereby the second melting peak temperature is raised to the range of 70 to 80xc2x0 C., thereby improving resilience.
This method, however, requires annealing the ionomer resin-based cover material as applied onto the core. Holding the cover material at a high temperature for an extended period of time causes the core to harden. This hardening can prevent formation of a ball having desired physical properties. Additionally, since the annealing needs expensive equipment and takes a very long time, it is undesirable to adopt the annealing step in the golf ball manufacturing process.
An object of the invention is to provide a golf ball which is improved in resilience without a need for annealing the cover.
The invention is predicated on the following discovery. By adding a metal salt to an ionomer resin and heating and mixing them, a heated mixture is obtained. When the heated mixture is measured by a differential scanning calorimeter, a first melting peak and a second melting peak appear at different temperatures. Quite unexpectedly, a very high resilience is exerted when a heated mixture exhibiting such a crystal melting behavior that the difference between the first peak temperature and the second peak temperature up to 30xc2x0 C. is used as the cover material of a golf ball.
More particularly, as is known from the model proposed by Longworth et al, the solid structure of an ionomer resin consists of three phases, crystalline zones 1 of polyethylene, amorphous zones 2 and ion cohered zones 3 as shown in FIG. 1 (See R. Longworth and D. J. Vaughan, Nature, 218, 85 (1968)). The ion cohered zones are composed of cations, carboxyl anions, and non-dissociated carboxyl groups and known as ion clusters. It is also known that the rigidity, toughness and excellent other properties of ionomer resins originate from ion neutralization (see Hirasawa, Kobunshi Kako (Polymer Processing), 473, 1978). It is then believed that the state of ion clusters largely affects the properties of a golf ball.
We investigated the influence of the ion cluster structure in an ionomer resin on the resilience thereof and the flight performance of a golf ball using that ionomer resin as the cover. It has been found that a golf ball having improved resilience is obtained when the cover is formed of a heated mixture of an ionomer resin and a metal salt which exhibits a crystal melting behavior that the difference between first and second peak temperatures is up to 30xc2x0 C.
Accordingly, the invention provides a golf ball comprising a core and a cover of at least one layer enclosing the core, at least one layer of the cover being formed of a cover material comprising a heated mixture of an ionomer resin and a metal salt, the heated mixture exhibiting such a crystal melting behavior that when measured by a differential scanning calorimeter, the difference between a first peak temperature and a second peak temperature is up to 30xc2x0 C.